Since no one has jumped in...here goes. This is somewhat like the blind leading the blind, but here we go.
I do not own an I-12, but I do have a Stoeger M2000, and I understand that they are very similar.
Not sure what you are refering to when you say "knob", but the procedure for the M2000 is as follows:
1. With the bolt locked open, remove the magazine cap by unscrewing it.
2. remove the forend and barrel together as an assembly.
3. While holding the bolt operating handle to prevent the bolt from slamming, release the bolt and gently allow it to slide forward.
4. pull the bolt operating handle (the curved giddygapper protruding from the bolt (it may be tight, if pliers are required, wrap it with cloth first to prevent scarring)
5. from the bottom of the receiver depress the shell elevator and reach into the action and depress the shell latch.
The bolt carrier and bolt should slide right out.
Sounds a lot harder than it really is. I can have mine out in about a minute.
